1. Field of the Invention
The present invention relates to a method for removing a resin mask layer and a method for manufacturing a solder bumped substrate, when solder and in particular solder bumps are formed through resin mask method in surface mounting of electronic parts on electronic circuit substrates, using a solder precipitating composition.
2. Description of Related Art
In recent years, as electronic devices have smaller size and lighter weight, electronic parts have more pins and narrower pitch. Conductor patterns as well have a larger number of conductors formed at very small intervals in a narrow range, which means that fine pitch becomes widespread. This is why mounting method using solder bumps is widely employed to bond electronic parts to electronic circuit substrates, instead of conventional wire bonding. Via-on-pad structure where via holes are prepared on pads in a substrate is also widely employed in terms of wiring design.
As for solder bump forming method, normally employed is resin mask method using a solder paste that is a mixture of solder powder and flux. That is, through conventional resin mask method, a resin film is formed on a substrate having electrodes, and opening parts to expose electrode parts are formed by development process. Then, a solder paste fills the opening parts, and heating process is performed to melt the solder paste, thereby forming solder bumps. Lastly, a resin mask layer is removed.
Through the above resin mask method, however, since heating process (normally at 200° C. or higher) is performed to form solder bumps, resulting in a resin mask layer that is more resistant to separation, it is difficult to completely remove the resin mask layer. Meanwhile, the residue of a resin mask layer on a substrate might have negative effects on subsequent electronic packaging.
As a resist stripping agent, a strong alkaline solution is generally used. However, a substrate soaked in such a strong alkaline solution has a possibility to damage solder resist and solder bumps on a substrate. This might cause a problem with reliability of solder connection.
Regarding a method for removing a resin mask layer, Japanese Unexamined Patent Publication No. 7-288372 discloses the use of an organic alkali aqueous solution, especially monoethanolamine as a stripping agent for alkali-soluble resist. Japanese Unexamined Patent Publication No. 6-250403 discloses that a strong alkaline aqueous solution including triethanolamine is used as a stripping agent for plating resist that is formed on a substrate by water-soluble resist. As for resist separation disclosed in these publications, however, heating process at a high temperature is not performed before separation process. Therefore, resist separation is considered relatively easy.